Polytetrafluoroethylene (PTFE) homopolymers and related modified PTFE polymers have exceptional stability to light, heat, solvents, chemical attack and electrical stresses, conferring desirable properties to articles made from these polymers. But because of the inability to melt process these polymers and the difficulties associated with solution processing, it is very difficult to spin or shape them by conventional methods. Therefore other processes have been developed for preparing fibers of PTFE homopolymers and modified PTFE.
Dispersion spinning is one method developed for producing shaped articles such as fibers from fluorinated polymers. Non-melt processible fluoropolymers may be successfully spun from a mixture of an aqueous dispersion of fluorinated polymer particles mixed with a solution of a suitable matrix polymer. An intermediate structure is formed when this mixture is contacted with a suitable coagulation bath. Although the intermediate structure is mechanically sound, a final, sintered structure is generally formed by heating the intermediate structure to a temperature sufficient to coalesce the fluorinated polymer particles. On sintering the matrix polymer decomposes to form volatile gases and a carbonaceous residue.
One method used to shape or spin polytetrafluoroethylene and related polymers is to spin the polymer from a mixture of an aqueous dispersion of fluoropolymer particles and viscose, where cellulose xanthate is the soluble form of the matrix polymer, as is taught in U.S. Pat. Nos. 3,655,853; 3,114,672; and 2,772,444. However, the use of viscose in fiber forming suffers from serious disadvantages related to cost of manufacture, production time and environmental hazards. Alternatives to viscose forming have been developed and most recently a process using cellulosic ethers with a uniform degree of substitution of the matrix has been fully described in U.S. Pat. Nos. 5,762,846 and 5,820,984.
Despite recent improvements, the prior art processes are still plagued by spinning compositions with insufficient shear stability. Because of the formation of PTFE agglomerates during mixing and pumping of the spinning composition, known processes employ filters at the spin pack to prevent pluggage of the spinnerts. With known processes the volume of agglomerates is significant, requiring frequent filter changes with the attendant material waste and low fiber production rates and yields caused by filter changes. In addition, processes using known spinning compositions are subject to an unacceptable number of fiber breaks during sintering at desirable spinning speeds.
It would be desirable to provide a spinning composition and a process for producing high tenacity fluoropolymer fibers in which the spinning compositions have the ability to withstand high levels of shear during processing, thereby reducing levels of agglomerates. It would also be desirable to provide a spinning composition and a process for producing high tenacity fluoropolymer fibers which decrease the number of fiber breaks during sintering.